Air quality concerns have resulted in the regulation of oxides of nitrogen from fuel burning equipment. This has given rise to a number of techniques and burner improvements to lower oxides of nitrogen emissions. Flue gas recirculation, fuel lean pre-mixing of the oxidant and fuel, fuel staging, and air staging are all techniques readily applied by those skilled in the art of combustion.
Fuel staging is a method wherein a small amount of primary fuel, or ignition gas, is used as an ignition source for a secondary fuel stream that would not otherwise combust. The secondary fuel stream can entrain flue products from the furnace resulting in lower flame temperatures and subsequently lower emissions of oxides of nitrogen.
Methods and apparatuses have also been developed wherein several of these techniques have been employed. For example, the prior art utilizes a fuel lean pre-mix in a staged manner where the lean pre-mix entrains flue gases prior to combustion and is simply ignited by ignition gas. While these methods and apparatus have achieved some success in reducing oxides of nitrogen, they have often compromised desirable attributes of burners with standard emissions of oxides of nitrogen.
For example, fuel lean pre-mix systems require higher fuel pressures to overcome the pressure exerted by the combustion air into which the fuel is injected. Furthermore, it is well established that burners that employ pre-mixture of the fuel and oxidant are more likely to acoustically couple with the combustion chamber, resulting in damaging and dangerous harmonics and pulsations.
The U.S. Pat. No. 5,195,884 to Schwartz et al. describes a low oxides of nitrogen burner. This burner has a tile 30 with a centered venture tube generally extending vertically. The tile has slanted walls upon the perimeter generally above the outlet of the venture tube. This burner operates upon a mixture of fuel gas and air sent through the venture and a secondary gas supplied upwardly upon the exterior of the slanted walls. A damper regulates air flow into the burner.
The U.S. Pat. No. 6,565,361 to Jones et al., where Jones is the applicant, shows a method for burning fuel with low formation of oxides of nitrogen. This method introduces pressurized air and primary fuel into a burner of frusto-conical shape. The walls, slanting inward, of the burner, compress the flue gases discharged from the combustion of the primary fuel. This method then introduces a secondary fuel air mixture upon the exterior of the walls of the burner at a velocity. The secondary fuel air mixture then blends with the flue gases for combustion outside of the walls of the burner.
Thus, there is a need for improvements in the methods of combustion resulting in low emissions of oxides of nitrogen.